Acoustical panel



Dec. l, 1959 c. w. LEMMERMAN 2,915,135:

AcoUs'rIcAL. PANEL Filed Sept. 12, 1957 INVHVTOR.

CARL u! LEMMERMAN TORNEYS United States Patent() ACOUSTICAL PANEL Carl W. Lemmerman, West Hartford, Conn., assignor to C. W. Lemmerman, Inc., Hartford, Conn., a corporation of Connecticut Application september 12, 1951, serial No. 683,536

7 claims. (ci. 181-33) This invention relates to an acoustical panel of novel and improved construction particularly adapted for use in constructing jet engine test cells or mufllers.

Where it is desired to attenuate sounds of mixed frequencies as they are passed through an enclosure,l effective attenuation or absorption of the higher frequencies may be accomplished without the use of particularly large or bulky structures by the provision of sound absorbent materials such as glass wool on or in the walls of the enclosure. The lower frequencies of sound will, however, not be absorbed by the sound absorbent material; and, normally, effective attenuation of low frequency sounds with their longer wave lengths brings to mind the use of relatively large and heavy attenuating structures. However, where the attenuating structure is in the form of a jet engine test cell on shipboard or a portable jet engine muler, a large and heavy structure is, ofcourse, extremely undesirable, if not impractical.

One type of acoustical panel which, while satisfactorily attenuating mixed frequencies of sounds, is of a size and weight which makes it adaptable for use in the construction of structures and, as described, comprises a pair of spaced apart stacked perforated plates, one of which forms the operating face of the panel, and a third plate which may be perforate or imperforate spaced inwardly ofathepanel in stacked relation to said pair of plates. A quantity of sound absorbent material is disposed between the second and third plates. In such a panel the perforated plates are of dimensions and the perforations of a size and spacing, such that the perforated plates will be vibrated by low frequency sounds engaging thereagainst so as to act as resonators to attenuate the lower frequency sounds'. The higherfrequencies of sound will pass through the perforations for attenuation and absorption by the sound absorbent material therebeyond.

Where such a panel is used in an environment such as jet engine testing, wherein the sounds are carried by high temperature gases traveling at very high velocity across the operating face of the panel, there is a tendency of the gases to erode the sound absorbent material and reduce the efficiency thereof. In some instances where the gas velocity is particularly high, the sound absorbent material will frequently be blown from the panel or at least will have a greatly reduced service life. In order to resist such erosion or blowout of the sound absorbent material, it is a practice to place a quantity of acoustically pervious gas ow resistant material such as steel wool or wire mesh between the two inner perforated plates. Such material is referred to by those skilled in the art as scrubble.

It has been found in practice that the scrubble is particularly difcult to pack to the uniformity of distribution and relatively high density desired for optimum protection of the sound absorbent material. Further, while a panel, such as described, provides a satisfactory low frequency sound attenuation, it is, of course, always desirable to improve the low frequency performance of the panel.

tion characteristic of the panel without increasing the size of the panel and without a substantial increase in the weight of the panel.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingly consists in the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.

Fig. l is a cross-sectional view of an acoustical panel constructed in accordance with the invention, and

Fig. 2 is a enlarged fragmentary perspective view of the panel of Fig. l.

With reference to the drawings, a panel constructed in accordance with the invention comprises channel side members 10 providing structural support and rigidity for the panel, as well as means for joining adjacent panels to form an enclosing structure. Secured along one side of the members 10, such as by welding, is a perforated plate 12 forming the operating face of the panel. Spaced from the plate 12 is a second perforated plate 14, also secured to the side members 10 by means such as welding to flanges 16 extending along and secured to the side members 10. As can be seen from the drawings, the spacing between the plates 12, 14 is quite considerably greater than the thickness of the plates. The third plate 18 is secured along the other side of the channel members 10 by means such as welding; and in the specific embodiment shown, the plate 18 is unperforated, although it could be perforated if desired. Disposed between thel second perforated plate 14 and the rear plate 18 is a quantity of sound absorbent material 20 such as glass wool. In operation, with mixed frequencies of sound carried across the operating face 11 of the panel by high temperature gases at a high velocity, the low frequency sounds will engage the perforated plates 12, 14, causing them to vibrate and act as resonators to attenuate the lower frequency sounds. The higher frequnecy sounds will pass through the perforations in the plates 12, 14 for absorption by the material 20.

In order to resist erosion and/or blowout of the sound absorbent material 20 in cases where the gases bearing the sounds are traveling at a particularly high velocity, a quantity of wire mesh or scrubble 22 is disposed between the plates 12, 14. In the specific embodiment and in accordance with the invention, the wire mesh is wound upon or wrapped around a perforated plate 24. For reasons which will be later apparent, it is preferred, particularly in the case of panels having a relatively large width, for example, approximately four feet, that a plurality of plates 24 wrapped in scrubble be provided, in substantially coplanar relation, intermediate the plates 12, 14. As can be seen in Fig. 1, in the specific embodiment, a plurality of plates 24 are used to extend 'the width as well as the length of the panel.

As shown in Fig. 2, the plate 24 is resiliently supported in spaced relation to the plates 12, 14 by the scrubble, and the scrubble disposed about the edges of the plates separates the plates 24. To assist in maintaining the plates 24 in position within the panel, it is preferred that a pair of anges 26, 28 be secured to the side channel members 10 and that these flanges be firmly engaged Patented Dec. 1 `1959- 3 in the scrubble on opposite sides of and spaced from the plates 24.

In the use of a panel constructed in accordance with the invention, while the scrubble 22 will perform in the usual manner as regards protection of the sound absorbent material 20, it further provides a resilient support for the perforated plate 24 so that when the plate is engaged by low frequency sounds it will vibrate, thus providing a low frequency sound attenuator. This added attenuation of the low frequency sounds provides the panel with considerably improved low frequency attenuation performance without any increase in size of the structure and with no appreciable increase in the weight thereof. The perforated plates 24 will also, of course, provide additional protection for the sound absorbent material 20 as it offers additional resistance to gas flow.

Further, the wrapping of the scrubble around the septum plates 24 provides an efficient and simple manner' to achieve uniform distribution of the scrubble as well as to provide a relatively dense packing. It has been found that densities in the neighborhood of 25 lbs. per cubic foot are possible with this construction.

Thus it can be seen that the provision of the perforated plate or septum 24 resiliently supported between the perforated plates 12, 14 by means of scrubble wound therearound provides a very practical means for uniformly packing dicult to handle scrubble to a high density as well as providing additional low frequency sound attenuating means for the panel. The size and number of plates 24 for any given size of panel are preferably selected to provide easy handling of the plates 24 and scrubble; further, the dimensions of the plates 24 are selected in combination with the size and spacing of the perforations in the plates to provide the frequency response desired.

I claim:

1. In an acoustical panel, a pair of perforated metal plates disposed in spaced apart parallel stacked relationship, a third perforated metal plate disposed between said pair of plates, and a quantity of sound absorbing gas ow resistant material resiliently supporting said third plate in spaced parallel stacked relationship relative to both plates of said pair of plates.

2. In an acoustical panel, a pair of perforated metal plates disposed in spaced apart parallel stacked relation, a quantity of sound absorbent material disposed alongside one of the plates of said pair, a resilient packing of acoustically pervious gas ow resistant material disposed between said pair of plates, and a third perforated metal plate embedded in said packing and supported thereby in spaced parallel stacked relation to said pair of plates.

3. Inan acoustical panel, a pair of perforated metal plates in spaced apart parallel stacked relation, a third perforated plate disposed between said pair of plates, and a quantity of metallic scrubble wrapped around and resiliently supporting said thrid plate in spaced apart parallel stacked relation to said pair of plates.

4. In an acoustical panel, a pair of perforated metal plates in spaced apart parallel stacked relation, a rear plate disposed in spaced stacked relation to said pair of plates, a quantity of sound absorbent material disposed between said rear plate and said pair of plates, a plurality of spaced perforated plates disposed between said pair of plates, and a packing of scrubble disposed between said pair of plates and resiliently supporting said plurality of plates in spaced apart stacked relationship to said pair of plates.

5. In an acoustical panel, a pair of perforated metal plates in spaced apart parallel stacked relation, a rear plate disposed in spaced stacked relation to said pair of plates, a quantity of sound absorbent material disposed between said rear plate and said pair of plates, a plurality of spaced apart perforated plates disposed between said pair of plates, and a plurality of layers of metallic scrubble wrapped around and resiliently supporting each of said plurality of plates in spaced relation to each other and in spaced parallel stacked relation to said pair of plates.

6. An acoustical panel comprising a plurality of side members, a pair of perforated plates supported in parallel stacked spaced relation on said side members, a third perforated plate disposed between said pair of plates, a packing of scrubble disposed between said pair of plates and resiliently supporting said third plate in spaced parallel stacked relationship to said pair of plates, and a pair of anges on one of the side members engaged in said scrubble and extending parallel to said third plate on opposite sides thereof land in spaced relation thereto.

7. An acoustical panel comprising a pair of perforated metal plates in spaced parallel stacked relation, a rear plate disposed in spaced stacked relationship to said pair of plates, a quantity of sound absorbent material disposed between said rear plate and said pair of plates, a plurality of perforated plates disposed between said pair of plates, and a plurality of layers of wire mesh wrapped around each of said plurality of plates and resiliently supporting said plurality of plates in spaced stacked parallel relationship to said pair of plates.

References Cited in the file of this patent UNITED STATES PATENTS 2,514,170 Walter et al. Iuly 4, 1950 2,674,336 Lemmerman Apr. 6, 1954 2,826,261 Eckel Mar. 1l, 1958 

